The present invention relates in general to humidity sensors, and more particularly to humidity sensors and methods for use thereof in metereological observations, such as by radiosondes or dropsondes.
Radiosondes are either lifted by a balloon from a ground station or dropped with a drogue from a high altitude platform and sense meteorological parameters pertaining to the atmosphere, especially pressure, temperature, humidity, wind speed and direction. These measurements are correlated with the altitude as the radiosonde descends or rises. Since balloon radiosondes typically rise at approximately 300 meters/minute, and drogue radiosondes, referred to as dropsondes, descend at twice that rate, it is essential that they respond rapidly to changes with altitude. However, relative humidity is one of the parameters for which sensors have been more susceptible to measuring lags.
Of the various instruments used for sensing relative humidity, chemical films are particularly suitable for radiosondes. The film, whose electrical resistance changes with exposure to moisture, is deposited on a structural substrate which provides external electrical connections to the film. Being low in mass, the film deposit responds rather rapidly to a change in the moisture content of the surrounding atmosphere; but the substrate, being massive relative to the film deposit, does not reach thermal equilibrium with the surrounding atmosphere as quickly. In fact, the temperature of the film deposit is more nearly the same as that of the substrate since they are so intimately connected. Consequently, the relative humidity indicated by the film deposit is that of the air at the substrate temperature rather than of the surrounding atmosphere. The physics of this phenomenon is such that the warmer the air, the more moisture can exist as a vapor in grains per unit volume before it condenses. Therefore, if the substrate is colder than the surrounding air, the indicated relative humidity will register higher than the actual or "true" humidity of the surrounding air.
This phenomenon can be extended to radiosondes. In standard atmosphere, the air is increasingly colder with altitude through the troposphere up to the lower boundary of the tropopause where the temperature is constant up to the stratosphere where there is an inversion. The air then begins to warm up with further elevation. In the case of a rising radiosonde, the thermal lag of the humidity sensor or "substrate error" is therefore in both directions, i.e. the substrate temperature is higher than the surrounding air temperature through the troposphere, and lower through the stratosphere.
On the other hand, dropsondes are usually dropped from an altitude below the tropopause where there is essentially only a continuous warming of the atmosphere. The thermal lag in the humidity sensor causes the substrate temperature to be always colder than the atmosphere. Hence, the indicated relative humidity will be higher than 100% if the surrounding air is saturated but the substrate temperature is lower. If the surrounding air is very nearly saturated, but the substrate temperature is lower, the relative humidity sensor might, and very often does, indicate 100% when, in fact, it is lower.
In a rapidly falling dropsonde where the substrate temperature stays colder than the air temperature, and the air is saturated or nearly saturated, the problem of accuracy is compounded. At high altitude launches, the temperature at the relative humidity sensor is usually very low, and as it descends the substrate temperature may be lower than the dew point of the air temperature at the lower altitude. Moisture in the air contacting the substrate and the film deposit then condenses on the sensor even though the dew point of the surrounding air is higher than the substrate temperature. Consequently, sensitivity to humidity is impaired due to water and/or ice accumulating on the sensor. If the temperature of the humidity sensor were always above the dew point of the ambient air, no water or ice could accumulate.
Humidity sensors of the prior art have no provision for maintaining the substrate temperature above the dew point while continuously measuring relative humidity. One such sensor, instead, allows for this condition by utilizing a mirror surface on a chilled substrate on which moisture condenses. When a photocell senses the condensation, a thermometer in the substrate measures the dew point from which the relative humidity can be determined according to well-known methods. Such a system is too expensive for routine use in radiosondes, and is incapable of frequent measurements because of the relatively long time interval required to clear the substrate by heating for the next reading.